1) Field of the Invention
The invention relates to a method of identifying mapping of a paper machine actuator in a paper making process, the method comprising forming a mapping model which takes linear and non-linear shrinkage of a paper web into account, and performing a mapping test to obtain a mapping test result.
The invention also relates to an apparatus for identifying mapping of a paper machine actuator, the apparatus comprising means for performing a mapping test to obtain a mapping test result, and means for forming a mapping model which takes linear and non-linear shrinkage of a paper web into account.
2) Description of Related Art
In a continuous paper making process, quality parameters measured in the cross direction of a paper web are controlled mainly using actuators arranged in the cross direction with respect to the paper direction. The paper quality parameters are measured with dynamic or static measurement devices, which measure the paper web in the cross direction. The cross-directional measurements are vectors which are called profiles. These profiles are controlled with actuators, which can change the shape of a measured profile. Controlling of the profile requires information on where and how each actuator affects the measured profile. The relation of the cross-directional location of the actuators to the location of the measurement devices is called mapping, and the process or method by which the relation of the cross-directional location of the actuators to the location of the measurement devices is determined is called a mapping test (thus reference herein to “mapping” will be understood to involve a mapping test procedure or method). One example of this is the profile bar in the head box of a paper machine, whose position affects the basis weight of paper. The position of the profile bar is controlled with the measurement information obtained from measurement devices located at the dry end of the paper machine. It is desirable to exert influence on the basis weight cross profile to make it correspond to the shape of the target profile as accurately as possible. The target profile is usually straight, but in some cases it is desirable to increase or reduce the basis weight of the edges of the web to produce paper with as uniform quality as possible. Uniform quality is obtained when the mapping of the measurement of cross-directional control is aligned with the mapping of the actuators.
The further away the actuators and the measurements are from one another in the direction of the paper web, the more difficult it is to align them. The reason for this is that the paper web usually also moves in the cross direction during the paper making process. In addition, the paper shrinks in the cross direction of the paper web. The shrinkage can be divided into linear shrinkage and non-linear shrinkage. A model of mapping consists of a model for cross-directional shift and of a model for shrinkage.
The mapping model may be static or dynamic. In the static case, mapping is modelled using a step response test, and a table showing the correlation between the actuators and the measurements is formed from the test result. This correlation table is used even though the process would change. In the dynamic case, the position of the paper web edges is measured continuously and the model is updated dynamically as the edge information changes. Mapping can also be implemented adaptively, i.e. the mapping model is tuned at the same time as it is used.
The mapping model is usually modelled using a step response test when the control is in the manual mode. In that case the step response test is performed with a few actuators. In the step response test the actuators are moved either manually or automatically from one position to another, which provides a response which is seen in the measurement profile and which indicates the shape and location of the actuator response. The response locations determine mapping of the control, after which the correlation model of mapping is amended to conform to the result provided by the test.
The problem associated with prior art solutions is that the model of mapping has to be corrected manually after an automatic mapping test. The mapping error is obtained from the test results by comparing the result with the current model. If there are errors, as usual, it is difficult to find out which part of the multi-part mapping model contains errors. In that case the mapping model may be corrected with an erroneous parameter, which leads to an unsatisfactory final result. For example, the shape of the non-linear shrinkage profile may change between different lines, and in the case of a new line mapping is no longer in order because the shape differs from that of the shrinkage profile used in the model. Alternatively, the mapping model error can be corrected with linear shrinkage even though the error had been caused by non-linear shrinkage. In that case, the level of cross-directional control decreases as the process changes and it may be necessary to perform the mapping test and correct the error again.
Fu, C. Y., Nuyan, S., Bale, S., CD Response Detection for Control, Proc. TAPPI PCE&I '98, Vancouver, Canada, pages 95–106, March discloses how both the movement of actuators and signal processing as well as analysis of the test result can be automated. Metsälä, T., Shakespeare, J., Automatic Identification of Mapping and Responses for Paper Machine Cross Directional Control, Control Systems '98, Porvoo, Finland teaches that actuators can also be controlled with inputs instead of state changes. In that case actuators usually need to be controlled so accurately that the control has to be automated and performed by software.
U.S. Pat. No. 5,539,634 discloses a mapping method for reducing the disturbing effect of the state change test signal on the paper to be manufactured by using a pulse sequence as the test signal. The detector uses machine directional noise calculated using profile measurements.
U.S. Pat. No. 5,400,247 discloses a method which comprises determining an actuator resolution decoupling matrix for the controller by first saving the controller's actuator resolution control profile when the process is controlled, and by calculating its effect on the measurement profile with the matrix which does not include decoupling. Approximately at the same time the measured profile change is saved and decoupling is eliminated from it using the decoupling matrix, which is changed as these two signals are minimized. Using recursive identification, the decoupling matrix can be modelled adaptively. The solution relates to identification of decoupling, but does not define mapping of actuators and measurements.
D. Gorinevsky, M. Heaven, C. Hagart-Alexander, M. Kean and S. Morgan, New algorithms for intelligent identification of paper alignment and nonlinear shrinkage, Pulp & Paper, Canada, 1997, pages T209–T214 discloses a method for determining mapping and non-linear shrinkage. The solution comprises correlating the predicted change of the actuators with the actual change, and thus test results can also be obtained from the measurement resolution profile. The solution comprises optimising alignment of two parameters of linear mapping by adjusting the predicted change and the actual change to each other as accurately as possible. The solution requires matrixes the size of which may be even 800 * 100, for which reason the method requires a considerable amount of calculation. In addition, the solution comprises generating a shrinkage profile using the inference rules of fuzzy logic.
U.S. Pat. No. 5,400,258 defines a mapping method which comprises filtering the result of the step response test by correlating the vector of the test actuator with the result vector. By using this pattern identification algorithm, noise can be reduced in the test result and mapping points found out. The method employs a measurement profile which comprises as many zones as there are actuators. The resolution of the measurement profile thus corresponds to the actuator resolution. As the result of the mapping test, a shrinkage coefficient profile is calculated, which is used for making the measurement profile to correspond to the actuators by calculating the coefficients of the shrinkage coefficient profile as a relation of the shrinkage of actuator zones to the total shrinkage. Any errors in mapping are corrected by changing the shrinkage coefficient profile. For example, if the error is in linear shrinkage, it is corrected in the shrinkage coefficient profile, which will no longer show the real physical non-linearity of shrinkage. Furthermore, the shrinkage profile is determined only by calculating it from the test results, in which case it is assumed that the result points are completely correct. If the result points have been defined incorrectly, which is rather common in processes in which the actuator responses are rarely identical, the shrinkage coefficient profile will also contain errors, and thus the physical non-linearity of shrinkage may be modelled incorrectly.
An object of the present invention is to provide an improved method and apparatus for identifying mapping between actuators and corresponding measurement points.